PROJECT SUMMARY We have recently identified a novel p53 pro-survival signaling pathway involving upregulation of p53 target genes or DNA damage checkpoint genes, which can influence the balance between cell death and arrest/senescence in a p53 dependent DNA damage response. We have identified the RhoE gene as a novel transcriptional target of p53 and DNA damage. We found that DNA damage triggers actin depolymerization, resulting in stress-fiber disassembly through p53-dependent RhoE activation. RhoE is a recent addition of the small GTPases superfamily and has the strongest homology to RhoA, B, and C. Despite this high homology, RhoE has several unusual properties which distinguish it from the other Rho proteins. Unlike other Rho proteins, RhoE seems to be only Rho family protein transcriptionally responsive to DNA damage that can induce loss of actin stress fibers. We hypothesize that RhoE protein are essential for survival of cancer cells with wtp53 by suppressing the activity of proapoptotic proteins while enhancing the function of prosurvival proteins in response to the exposure of DNA damage. The main goal of this proposal is to understand the unusual roles of a small GTPase, RhoE, as a novel target gene of p53-dependent DNA damage and to translate our basic understanding of the cell survival function of RhoE into apoptosis for human cancer cells including breast cancer cells. Although the biological role of RhoE in DNA damage response pathway has not yet been defined, our preliminary data demonstrating transactivation of the RhoE gene by p53-dependent DNA damage, its potential role as a key organizer of the actin cytoskeleton, and its exceptional role in DNA damage-induced cell survival are intriguing to us. Accumulating evidence suggest that an aberrant signaling through Rho proteins is closely associated with various steps of tumorigenesis and carcinogenesis. In this application, a combination of biochemical and genetic approaches will be used to (i) delineate the functions of RhoE in p53 dependent cellular outcomes and DNA damage stress response; (ii) Identify and characterize RhoE Interacting partners, which will define a molecular pathway that affects RhoE-mediated cell survival and p53-dependent cell death in response to genotoxic stress; iii) determine if ROCK 1 is an essential upstream regulator for JNK-mediated cell death signaling; and (iv) define the role of RhoE in DNA damage checkpoint signaling and cell survival signaling using in mouse knockout approach.